Data communication systems, such as cable systems, typically include a head end which transmits data to a plurality of subscribers over a cable system. Typically, the cable system is at least partially buried and has a cable main trunk carrying data directly from the head end, cable branch lines branching out of the main trunk, and cable subscriber lines carrying data between the cable branch lines and the subscribers. Considerable labor is required in running subscriber lines from cable branch lines to subscribers, particularly for those subscribers who are located at distances such as 1,000 feet or more from the cable branch lines.
Instead of running subscriber lines from cable branch lines to subscribers, transmitters could be located periodically along the cable branch lines in order to transmit data over the air between cable branch lines and subscribers. Thus, the substantial labor which is necessary to connect a subscriber to a cable branch line is materially reduced. However, care must be exercised in locating such transmitters. For example, if a subscriber is covered by only one transmitter, there may be areas within the premises of the subscriber where reception is poor.
The possibility of poor reception can be lessened by locating the transmitters sufficiently close to one another that the premises of each subscriber is covered by two or more transmitters. Unfortunately, because each transmitter operates at the same carrier frequency, and because of the variable distances between a subscriber's premises and the transmitters that cover the subscriber's premises, the same data may arrive at a reception site within a subscribers premises at different times and with different phases. As a result, interference, referred to herein as ghosting, is produced.
If signal amplitude versus frequency of the received signal at a reception site in a subscriber's premises covered by two transmitters is graphed, an interference pattern can result. In the case where the reception site is located at an equal distance from both transmitters, the interference pattern has the shape shown in FIG. 1. This interference pattern may be sometimes referred as 100% ghosting. In this interference pattern, the signal amplitude of the received signal is characterized by periodic, sharply defined nulls at which the signal is substantially undetectable, particularly in the presence of noise. That is, noise in the channel establishes a signal detection threshold above the horizontal axis as viewed in FIG. 1, such that any frequency components of the transmitted signal near or at the nulls will be difficult or impossible to detect because the signal to noise ratio at these points is too low. Moreover, when the received signal is processed through an equalizer, the signal to noise ratio can worsen, making signal detection even more difficult.
It is known how to adequately receive signals in the presence of white noise. For example, trellis encoding and Viterbi decoding may be used to encode and decode transmitted data adequately when white noise is present, because this type of coding and decoding performs well under white noise conditions. Unfortunately, trellis encoding and Viterbi decoding do not work particularly well in the presence of non-randomly distributed noise, such as may be present in an environment experiencing 100% ghosting.
The present invention is directed to a coding and decoding arrangement which is particularly useful in a single frequency, multi-transmitter network in which 100% ghosting is present.